Gas turbine engines are used for a variety of purposes where large amounts of power are required at high efficiency. While gas turbine engines are commonly used in aircraft transportation because of these characteristics, they can also be used to provide power for other applications including marine craft and land-based electrical power generation. These engines are comprised of a series of stages that are operated on a single power shaft or can be operated using a plurality of shafts to efficiently provide high output energy. Typically, these stages include, in the order in which incoming air passes through them, a low pressure compressor, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. See commonly assigned U.S. Pat. No. 5,323,604 to Ekstedt et al, issued Jun. 28, 1994.
Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions, especially for industrial and power generation applications. Nitrous Oxide (NOx), which is a precursor to atmospheric pollution, is generally formed in the high temperature regions of the gas turbine combustor by direct oxidation of atmospheric nitrogen with oxygen. One technique for the reducing of gas turbine emissions is “lean premix” combustion, where the fuel is burned at a lower temperature. By “lean premix,” it is meant that the fuel/air mixture contains more air than is required to fully combust the fuel, or an equivalence ratio of less than one.
A combustor utilizing this “lean premix” concept is commonly referred to as a “dry low emissions” combustor. One such “dry low emissions” combustor is disclosed in commonly assigned U.S. Pat. No. 5,323,604 (Ekstedt et al), issued Jun. 28, 1994, and comprises three separate or annular domes, with each dome having disposed therein a plurality of circumferentially spaced mixers for uniformly mixing air and fuel. To provide low fuel-air ratios at maximum power where the fuel flow rate is high, the triple annular or dome Ekstedt et al combustor utilizes approximately 80–90% of the total combustion air in the dome. This is achieved by having a large number of fuel/air mixers to accommodate high dome flow so that the combustor is able to operate in a temperature range that minimizes NOx, carbon monoxide (CO), and unburned hydrocarbons (UHC). See also commonly assigned U.S. Pat. No. 5,197,278 (Sala et al), issued Mar. 30, 1993 for a double dome combustor of the “dry low emissions” type.
To utilize approximately 80–90% of the total combustion air in the dome, “dry low emissions” combustors such as the Ekstedt et al combustor employ a diffuser to diffuse the compressed inlet air stream to be supplied to the combustor. One such diffuser used with the Ekstedt et al combustor is disclosed in commonly assigned U.S. Pat. No. 5,335,501 (Taylor), issued Aug. 9, 1994, and comprises a plurality of splitter vanes to better spread the airflow radially. The Taylor diffuser is also typically equipped with a plurality of compressor bleed ports that extend radially outwardly from the diffuser. In those instances where the “dry low emissions” combustor (e.g., Ekstedt et al combustor) is not operating at peak power or full load (e.g., where the operation of the combustor is scaled or “throttled” back), a portion of the air entering the Taylor diffuser is usually diverted or “bled” through these bleed ports to maintain stable combustion, i.e., smooth transitions between combustor modes without excessive acoustics, and without lean “blow out.”
This “bleed air” can typically represent up to about 12% of the total compressor inlet air stream to the diffuser. While “bleed air” is necessary to maintain stable combustion in “dry low emissions” combustors of the Ekstedt et al type, it also considerably increases the heat rate (i.e., lowers thermal efficiency) of the combustor. This causes a significant fuel burn penalty for “dry low emissions” combustors when “bleed air” is required, compared to other combustors that do not require “bleed air.” Accordingly, it would be desirable to be able to improve the thermal efficiency of such “dry low emissions” combustors that require “bleed air” during those periods when not operating at full load, i.e., when operating at a partial load or in a “throttled” back state. It would further be desirable to be able to improve the thermal efficiency of such “dry low emissions” combustors while keeping emissions within desired limits while operating in such a partial load or “throttled” back state.